Here are some screenshots for Cagayan de Oro City:
I am not so sure if this is the flooded area, but based on videos on the internet, the features resembled that of the actual flooded area. Well, this rendition is just for discussion purposes on how land use can affect hydrology and how urban planning can render populace vulnerable to risk. Take for example this image above, it shows the topography like a trough. Unfortunately, most of the residences are within this trough.
Going back to our larger picture, we can actually see two watershed draining to the vicinity of C. We added point E because it seems D have its own pathway but still in the vicinity of C. Here, we illustrated that floodwaters coming from A and B are essentially choked at C (like a trough) and eventually flush out water (illustrated as a three pronged arrow). Floodwaters from D would eventually settle on the E floodplains. By the looks of it, it may not be as destructive as in point C.
So we have here (see image above) a situation that a large watershed drains to a small floodplains (marked CDO).
It is wide that point A to C is approximately 50 km and B to C is around 20 km. By the looks of it point A is already in Bukidnon.
Examining satellite images, we can see that land uses can be agricultural fields in point A and B. Looking closer at point A, it can be gleaned that it is of intensive agricultural land use as evidenced by its brown patches. That is also true with slopes at point D. Although at point D, we cannot discount possible logging activities there due the presence of forest patches. What is crucial here is to know (ground truthing) the actual land use activities on point A and B. Agricultural activities that need to be reviewed are vegetable and corn farming - considered to be erosive agricultural land practices. Are agricultural activities in those areas employ some sort of a soil and water conservation to arrest runoff? If farmers do not consider this, dwellers in C will be forever at the mercy of floods. This large watershed may encompass several municipalities in Misamis Oriental and Bukidnon. Negotiating proper land use to protect CDO might be an uphill struggle but worth the try.
Zooming in on C, you will find several property development. Locally, if these subdivisions are constructed in such a way that drainage connects to the main river, it will add to the problem. This is particularly true if streets are paved and residences lawns are paved as well. I suggest that they employ some sort of water retention facilities (in agricultural engineering we called it bioretention) to contain storm water to at least delay the runoff towards to the river. Or design houses with rainwater harvesting or green roofs. For point C, they have no choice but to start building houses with second floor to adapt to the situation. Our forefathers are smarter than us. They built houses on stilts!
More or less, the Davao City incident has some pecularities with CDO case:
But unlike CDO, the Davao flashflood was triggered by localized rainfall on a smaller watershed. But the same mechanism is still in place and that is the presence of a "choked point". Looking at satellite imagery, the watershed were possibly plantation crops (banana, fruit trees). Unlike that of CDO, this is less erosive provided planted properly. What I mean by planted properly is that it should consider the contour and slope. But looking closely (please try Google Earth) some of this plantation crops was planted along straight lines. In this case, runoff is still high.
I could not discount the possibility of the contribution of property development or quarrying on top of the Matina floodplains. But I guess the biggest contribution comes from the agricultural plains in the north. A larger watershed is needed to impound such large volume of water to flush the floodplains. I mean flush not just flash.
I just hope that this will help explain some phenomena we experience nowadays. Hope that policy-makers will consider hydrology in crafting their respective land use plans. Again, these illustrations are meant for discussion purposes. We still need to verify on the ground the actual land use before we could definitely give conclusions.
Please read also: using rainfall to save yourself from floods and typhoon warnings: switching from wind to rain . If you think this is helpful, you are free to share.
